I don't think you can really compare without some pretty significant knowledge of people's medical history, that's the problem with these discussions.
The money element is unusual of course and does raise the question whether someone might be looking to exploit Spears, but in another sense it's not the point - people who are wealthy can still become seriously mentally ill. And then, a very wealthy person without capacity may be more at risk of being exploited if they have control of their own affairs, too, by people looking to take advantage of them.
I don't think being able to work in terms of appearing on stage etc is really indicative of whether she's able to take care of her business or financial affairs. The issue to some extent seems like it might be around what happens when she's off her medications, though people can sometimes be functional in some areas but not others, and it isn't therefore desirable to stop them functioning in the areas where they are more able.
As far as the IUD, that seems unfortunate, but as a pp said, might be related to her medication, or it may be that it's felt that she's particularly vulnerable to sexual exploitation in some way and it allows her some autonomy in that regard.
It's good that she can bring this before a judge, but the idea that the general public has the information, or should have it, that would give us the knowledge to have an informed opinion? I just don't see how that is the case.